Rectifier manufacturing process and products obtained thereby



Sept. 25, 1951 A. VON. HIPPEL ETAL 2,568,780 RECTIFIER MANUFACTURING PROCESS AND PRODUCTS OBTAINED THEREBY 2 Sheets-Sheet 1 Filed March 1, 1947 oww Gu INVENTORS 14/?7' HUI? VON H/FPEZ moan/v51? c 5100M ATTORNEY Sept. 25, 1951 A VON HPPEL ETAL 2,568,780

RECTIFIER MANUFACTURING PROCESS AND PRODUCTS BTAINED THEREBY Filed March 1, 1947 O 2 Sheets-Sheet 2 l I l l l ion 200 300 400 .500 600 700 800 900 I000 I100 I200 a/a/ms 550 Pan L/ ER offlC/fl (/a/v H2 50 Paw/V7405 yfcl/mslvr 071/4250 M web 4M a/SELEN/l/M Patented Sept. 25, 1951 UNITED sTATEs PATENT ,t

Arthur. von" HippeLI, vWeston, and Mortimer 0. Bloom, NewtonpHighlandsirMass assignors to Federal Tel ephoneand Radio flo i sl lation, New York, N. Y., a corporation of Delaware Application March 1, 1947,'Seria:l'NU;'7313834 10 Claims; 1

This application is a continuation in partof copending application" Serial No. 496;?60 filed: by the-same applicants on--July'30, l943',iwhich inturn is a continuation in par tof application Seri'al -No.420,906filed November 29, 1941 by the same applicants, now abandoned;

- This invention relatesgenerally to improvements in processesfor the manufacture'of asymm'et'r-ic electrical conductors useful in rectifyinga-l't'ernating current and'in particular it is concerned with a novel 'pro'cessfor producing a new improved type of selenium rectifier. 7

According to one Of-"the" best of the presently practiced commercial processes, selenium rectifi'e'rs are manufactured'byapplying to a metallic base plate pulverised amorphous selenium to which has been added afconductivit'ypromoting agent,- heating initially 'tosoften or fuse theselenium preferably under pressure thereby caus-I ing "it to distribute itself as asubstantially uniform coating on theplate and then later con- (012 204-5)f V T vertingit*cornp1etely"from theamorphous 'to' the gray or metallic variety, applying ametallic conductor "as a-*counterelectrode; and thereafter electroforming to develop the rectify-ingqual-ities of the element. a

H Howeverthisprocedure, ent irely aside from the fa'ct that it isessentially a piece-work-and mata' continuous operation; presents" many prob lems that are very difiicult. to solve under usual conditions of operation even though highly skilled technicians are. employed and] arbitrary empirical standards of procedure and intermediatepro'ductsare-rigidly enforced. For example,"when a'pplyingthe pulver'isedi selenium to the base plate, eXtrerne'care must-be exercised to assure that it"is'distributed in' a substantially con-- tinuous and uniform layer upon the plate as otherwise, in the subsequent heating, the. softe'n'ed" or fused material may fail to coalesce with the result that in the finished rectifier the counterelectrode will make direct contact with the baselpla'teand-cause a short circuit. This operatioln'fislurther complicated by the contraction of the volume of the selenium when the. amorphous formis convertedto' the metallic form which also'can; produce discontinuities inthefilm unlessspecial precautions aretaken. Also under these conditions, the heating can cause superfi-' cialoxidation of the exposedarea of the base plate with the result that even if selenium is then applied tothis-area it will not adhere and malgesatisfactory electrical contact. When it is consideredethat the seleniumlayer in a rectifier element ismade very thin in order to minimize producing-a continuous and-uniform coatingace cording: to: this :present day practice demandsl a high degree of technical skill both in the vmanu facturingitselfi and also inthecritical examinae tioneof the intermediateproduct.,

Various r expedients, none wholly. satisfactory; have been-suggested to overcome. even partially the-shortcomings of-this priorart procedure. In, particular, v itrhas loeen suggested that the base plate could be initially-ooatedwith seleniumb'y condensing it thereon in vacuum This prone: dural modification Was intended to preventioxie; dation of thev baseplate, prevent. contamination of the-selenium layer. during the coating operation and, .in genera1,-.-to promoteethe manufac: ture ot a uniform product- However, this ,-proc; ess hasnot satisfied-i industrialneeds because it merely adds a costly. operating condition demanding. highly... skilled andv specialized techniqueeoperating inf vacuurr -rwith resultan increased cost of but without material im rov ment in product. Furthermore the elements thus coated with selenium must .still" beheat processed as heretofore to obtain the desired layeraof metallic selenium and-during these op: erations the difiiculties alcovementioned are therefore till encountered. A further, difficulty encountered .in theuabove described proces's lie's in the step. wherein v a conductivity promoting agent isincorporated. in. the. amorphousisele? nium; the problem presented'here essentially is tosecure. a homogeneous mixture; of the sele; nium and the agent, which cannot readily be obi tained' because most of the .agents now used-..are employed in almost; insignificant proportions; say about 20.millig-rams' fiag'entper grams ofselenium,-with resultant difficulty iiilpiodricing a uniforrn mixture...

In addition to the technicab problemsprese'rit; ed eby theseexisting. commercial processes which to-ameasure inhibit-large scale exploitatiornoi the seleniumrectifier by restricting its availability, r therectifiers thus} far marketed have not been entirely satisfactory as regardsuniformit Qf electrical properties andeiliciency as rectifiers; This lack; of; uniformity has; been traceable: n: r: ea re t variations i m ia turin Pr edu e-J mal ha k n ue 'to impurities in the selenium; inthe base-platepr t e v e tere ee d ria wwlli h PW? h wres nt tra s s m n t -as most 9 defy: detection may materially and advers ,y ffe th -i eer pert e 9f; he .6 m

{The electrical efficiency. or present-day selenium atria-ted rectifiers has not been considered wholly satisfactory due to the substantial amount of electrical energy thermally dissipated because of the relatively high resistance of the selenium layer.

The objects of this invention include providing a new, simple, easily operated and commercially practicable process for manufacturing selenium rectifier elements, free from the disadvantages above mentioned, and. which is capable of producing rectifiers having a uniformity of electrical characteristics and an eificiency of operation to a degree hitheto unachieved in commercial production.

In accordance with this invention, these ob-.

jects are achieved by directly electrolytically depositing metallic selenium upon the metallic base plate as the cathode durin'g' electrolysis of an aqueous electrolyte containing tetravalent selenium cations, preferably an aqueous solution of selenium dioxide or selenious acid. At least the initial period of this electrolysis is conducted at an elevated temperature for reasons hereinafter appearing. I'he selenium deposits obtained by practice of this new process are tenaciously adherent to the base plate and are substantially entirely composed of the metallic form of seleniumfree from the red form of the element.

Among the major advantages of this process are that it can be operated as a continuous process and the selenium can be deposited as an essentially uniform continuous layer upon nonplanar surfaces, neither having been practicable heretofore. V w

One of the advantages of this process which isjof special significance in the manufacture of seleniumrectifiers is that it permits variation of the physical properties of the deposit within a wide range and also it permits variation and unusual improvement of the rectifying properties in terms of forward and reverse electrical resist-, ance. vFor example, rectifiers made from electrodeposited material wherein the selenium layer is approximately 0.003 inch in thickness, after electroforming, have passed 25 milliamperes per square centimeter at 1.1 volts in the more readily conducting direction and less than 0.15 milliampere per square centimeter at 18 volts in the less readily conducting direction, both measurements having been made at room temperature. This is in contrast to good samples of prior art rectifiers which passed 25 milliamperes per square centimeter at 1.1 volts in the more readily conducting direction and about 1 milliampere .per square centimeter at 18 volts in the less readily conducting direction, both measurements having been made at room temperature.

A further advantage of this novel process for producing selenium rectifiers is that the selenium deposit is invariablyobtained as a continuous film irrespective of how thin the deposit may be. This arises from the fact that the selenium film presents resistance to .electrical current flow during the electrolysis, hence it will be obvious that any discontinuities in the film would im-- mediately, constitute foci for further deposition of the selenium. By reason of this phenomenon, rectifiers utilizing extremely thin selenium layers may be made which have a correspondingly very low resistance to current flow in the more readily conducting direction. For example, rectifiers square centimeter at 18 volts in the less readily conducting direction, both measurements having been made at room temperature.

An additional advantage of the process according to this invention, which is of particular importance in the manufacture of selenium photocells, is that the light absorptive qualities of the selenium may be altered by adjustment of the conditions of electrolysis, for example, the color of the deposits may be varied from light gray to deep black with corresponding change in reflectivity for visible light from about at least 25% to 4% or less. This variation in appearance of the deposit is due to variation in the size and orientation of the individual crystallites which constitute the mat. The size of the crystallites may be varied from sub-microscopic dimensions up to a size readily observable with the naked eye and the orientation of the individual crystallites may be .varied from the condition in which substantially all of, the crystallites are C axis oriented normally to the supporting surface to a condition in which the C axis is far from normal to this surface.

A further advantage arising from practice of this process, which is of especial significance in the application of protective coatings, is that the deposit is corrosion resistant and is non-porous even though its thickness is less than about .0001 inch. It of course will be understood that by the term corrosion resistant is meant resistance to any corrosive action to which metallic gray seleniumis corrosion resistant.

To facilitate a better understanding of the presently invented process, certain data pertaining thereto are embodied in graphs illustrated in the accompanying drawings.

Figure 1 is a graph illustrating the relationship of temperature variation to the rate of transformation of amorphous selenium to nuclei of gray crystalline selenium. 1

Figure 2 is a graph indicating the relationship I of selenium dioxide concentrationin the electrolyte with the cathode efficiency for selenium deposition.

Figure 3 is a graph illustrating the relationship of maximum permissible current density in the electrolysis to the acid normality at difierent concentrations of selenium dioxide in the electrolyte.

In connection with the development of the process according to this invention it was found that at room temperature electrolysis of a selenious acid solution yielded a cathodic deposit of red seleniumwhich, as it increased inthickness, correspondingly increased in electrical re-, sistance until it formed an insulative coating on the electrode. An investigation of how a coating of amorphous selenium could be thermally or otherwise converted to the gray metallic crystal line form of selenium included an examination of the rate of this conversion in terms of temperature. The data derived from this investigation is summarized in Figure 1 of the drawings from which it will be evident that the maximum rate of conversion from the undesired amorphous form to the desired gray crystalline form occurs at, a temperature of approximately C. although with less satisfactory results somewhat higher or somewhat lower temperatures could be used. It was then discovered that if the initial amorphous layer had been thus converted to the metallic gray variety, further electrodeposition resulted in a thickening of this gray metallic deposit without any deposition of the red form of selenium. Forthis reason it isapparent that the optimum condition forypractice-of the process of However selenious acid also can: yield. water and tetravalent selenium ions:

. HzSeO3+4H+=Se+ +3Hz0 (4) Examination of the reaction coefflicienti' [H SeQ3][H ]4 makes evident that the thermodynamic concentration of the tetravalent seleniurnions in the reaction mixture will vary in the 4th power as the thermodynamic concentration of the hydrogen ion is changed and in the reciprocal third power as thermodynamic concentration of the water is altered, and in direct ratio as the thermodynamic concentration of the selenious acid is changed. Thus it will be perceived that in order to secure the desired high thermodynamic concentration of tetravalent selenium ions. the thermodynamic concentrationof the hydrogen ion and the thermodynamic concentration of the selenious acid are to be maintained at the highest practicable values, and the thermodynamicconcentration of water in the equilibrium mixture is to be reduced as far as possible.

From Equation 1 above it will be seen that addition of selenium dioxide to the aqueousmixture apart from the volume change which reduces the water concentration still further, reduces the water concentration by consuming one molecule of water per molecule of selenium dioxide added and also increases the selenious acid concentration to a chemically equivalent degree. In addition, high hydrogen ion concentration in the equilibrium mixture not only promotes the production of tetravalent selenium ions from'the selenious acid as in Equation 4. above but afidir tionally suppresses the dissociation as Equations 2 and 3 above. I

In harmony with these facts it is found, in accordance with this..invention, thatthe reaction of Equation 4 abovecan be utilized advantageous.- ly when determining conditions for the 15039- lytic cathodic deposition of selenium. Y

In this electrodeposition the electrolyte pref- -'erably comprises a "highly concentrated solution lowratio of tetravalent selenium-ionstohydrogen ions, thus limiting the current denSitleS Wh-iGl1 may be used without hydrogen evolution. This last condition, namely hydrogen evolutienat the cathode, is particularly undesirable :in rectifier manufacture because the bubbles of' gas are likely 'to-adhere to the cathode surface -"andinter fere with the formationota continuous ueposit. *Referring to Figure -2- of the-drawings, it will "be seen that the cathodeefficieney; i -e-., the ratio ode to the weight: of selenium theoretically-1 caepable-of being deposited onthe cathode during electrolysis increases with increase of. selenium dioxide concentration in the electrolyte until'ff-uli efli'ciency: is 'achieved, i. e-., the current is fully iitilized for converting the selenium from the tetravalent to.the zero-valent. state. This agreement'betwe'en-the theoretically and: the actually necessary current magnitudes clearly demon= strates that tetravalent selenium ions are uti'.-

"lized.

Under these conditions, the total cathode reaction-is:

. :produce' the desired form of selenium :It isrto be clearly. understood that upon. 6160-. trolysi's: of the electrolyte, the current is utilized either for depositingselenium or for causing hy;- drogen evolution, hence as. these factors are re- 'latedin the sense that their sum is unity, and the hydrogen evolution is undesirable when making theirectifi'er elements, one of the optimum conditions for practice of the presently invented :process; is the use of a sufficiently high concentration of selenium dioxide in the electrolyte "to assure 1.00% cathode efiiciency, i. e., no evolu- .tion of'hydrogen.

'Ehe conditions under which this desired full cathode efficiency may be achieved are made evident upon reference to Figure 3' from which it will be seen that the maximum permissible current density which .can be used'for selenium deposition without hydrogen evolution increases as the acid and selenium dioxide content of the electrolyte are increased until the solubility limit of "the selenium dioxide :in the acid. is reached. or until the acid reaches such concenttrations. that thereafter ionization progressively Idiminishes;

'..-Arf urther and --major factor affecting satisfactory operation ofthe presently invented process is. the temperature at which the electrolysis "is performed, as has been mentioned above in the discussion of Figure 1. Crystal nuclei which are created at a useful rate only in a limited tem-- --perature range and the rate of formation of .such. nuclei, in addition to their velocity of .g'rowth-,'.are functions of temperature. I *it'has-been. determined in connection with this -mvention that the .efiective generation .of: gray metallic selenium b nuclei formation. occurs at useful rates between temperatures of about 55 .C. and about 140 0., the upper and lower-useful: limits varying somewhat as other factorsare changed. The rate of formation of nuclei of the gray form passes through a maximum value at-a temperature in the neighborhood of. Cnto '1i00 C., and drops speedily on either side of this interval to relatively low values. When the other factors are suitably controlle the optimum temperature range is between about 75 C. and about C. If the bath is operated at temperatures. outside this range, 1. e. temperatures at which the transformation velocity from the red into the gray form is very low, any red material initially formed stays as such for a long time. When the bath temperature is maintained within the optimum range specified corresponding to rapid nuclei formation, any selenium deposited in the red form transformssufliciently rapidly into the gray metallic selenium that any material interference with'theelectrolytic process by the undesired-red "of' the weig-ht of selenium-'depositedon the cath-imaite1 ial-=is avoideds Moreover, oncetheinit-ially b'efore' immersing the cathode. this cathode is first immersed it is actually amorphous deposit is transformed to the desired gray crystalline form, either by operation in the optimum temperature range or otherwise, subsequent deposition can be carried on outside this, range without further formation of the undesired red product. At temperatures above the optimum range, this further'deposition can be carried on at higher than normal current densities without hydrogen evolution, but at temperatures below the optimum range, lower than normal current densities must be used if hydrogen'evolution is to be avoided.

Another factor affecting operation of this process involves both the selection of a suitable acid and'regulation of its concentration. The selected acid must be adapted for use in concentrations at which the desired concentration of hydrogen ions will be present in the solution and atithese'concentrations it must be free from any materialitendency to attack the base plate or theselenium deposit. -"The metal selected for the base plate will of course be an important factor in determining the suitability of particular acids. In general, the common strong inorganic mineral acids have been found to be preferable, sulfuric acid being the most satisfactory under usual conditions, although selenic acid is also satisfactory. Nitric acid, althoughsatisfactory in most respects, has

disadvantageous reactivity in high concentrations; Weak acids, such as the phosphoric acids,

may also be used, but since they produce low hydrogen ion concentrations and have high viscosities they are less desirable.

. "Excellent deposits can be obtained on plati- :num',; nickel, silver, antimony, steel and other metals; and it is preferred to choose the acid to be employed with regard to the metal used as a cathode so as to prevent attack on the cathode by the acid. Where it is necessary or desirable from the standpoint of speed or economy to employ, as the cathode, a metal which tends to react with the acid used in the bath, special precautions 'must be taken in initiating the plating in order to prevent the acid from acting on the cathode beforethe selenium plating is started.

In some cases such adverse action can be substantially avoided by applying the current as soon as the cathode is inserted into the bath, which'can be accomplished by connecting up the electrodes and turning on the current before im- 'mersing the cathode.

"Moreover, when the cathode metal is readily attacked by the acid, certain other precautions may be employed- One suitable procedure is to connect the current source in reverse polarity Thus, whenan anode and the positive hydrogen ions are kept away from themetal of the cathode.- A

thin. coating of a selenide, selenate (or possibly "some other compound) of the cathode metal appearsto be thus formed. Thereafter the current is reconnected in its regular polarity so that -thecathode is negative and plating commences. The thin protective coating appears to have no substantial deleterious effect, but does protect the metal from the action of the positive hydrogen ions which might attack it at the commence- -ment of plating, with consequent prevention of spitting or formation of a deposit of red amor- -phous selenium due to chemical reaction. For

instance,*a bath containing selenic acid for'the f-above-described brief reversecurrent treatment ao'f thejcathde may be used even if some'other.

ranges of variation of the main factors affecting,

the process will be evident from the drawings.

The term selenium coated article as used, herein embracesselenium rectifiers, and selenium photocells and objects bearinga protective coating of selenium of the type hereinabove described; by an electrodeposited selenium coating is meant a coating of the type obtained by practice of the vhereindescribed process, whether due to electrolysis or due to other phenomena occurring under the conditions of operation; and by tetravalent selenium ions is meant tetravalent selenium cations per se or a cation containing tetravalent selenium capable of discharging metallic selenium at the cathode upon electrolysis as above described. By the term electrolyte is meant a solution capable of conducting an-electric'current and of being decomposed thereby. A strong acid is one that is highly dissociated to give hydrogen ions in solution even at merely moderate dilutions, for example sulfuric acid. 7

To facilitate a clearer understanding of the matter ofthe present invention and how the process may advantageously be practiced, certain specific examples thereof follow but it is clearly to beunderstood that these examples are provided by way of illustration and not by way of limitation.

Example 1 An electrolyte is prepared by dissolving pure selenium dioxide in slightly more than an equal weight of pure water at room temperature, and the solution is then filtered using precautions to prevent contamination. The solution is then concentrated by heating and pure sulfuric acid is added in a quantity. such that the resultant electrolyte contains the following substances in substantially indicated proportions:

Selenium dioxide about 1225 grams Sulfuric acid (sp. gr. 1.84) about 370 ml. Water about 370 ml.

This electrolyte is placed in a container formed of a material inert to the electrolyte under conditions of use, erg. glass, stainless steel and the like, which is provided with a suitable protective cover for excluding contaminants and is equipped with such auxiliary apparatus as may be desired for supporting-the electrodes therein while immersed in the electrolyte. Preferably the container-also is provided with auxiliary apparatus for circulating the electrolyte therein during the electrolysis and also means for permitting temperature control ofthe electrolyte, particularly for maintaining it at a preselected temperature without significant variations.

The electrodes used in the electrolysis are fabricated from an electrical conductor which is inert to the electrolyte under the conditions of use, e. g. platinum or the like, and are cleaned by operations conventional in the electrochemical arts beforeguse, e. gacidyyashing, electroproceeds.

cleaning, picklingand he like, after which they are carefully rinsed and immersed in the electrolyte.

.Afterassembly ofthe apparatusia source of direct current .is connected with the .electrodes. This source-should becapable of Supplying sufiicient current to maintain a current density ofat least 100 amperes per square foot of cathode surface under the conditions of electrolysis. While maintaining the electrolyte temperature at about 100 C.,the electrolysis is'begun at a currentdensity of Y ,amperes per square foot of cathode surface which, after about 10 minutes, is; increased to about 100 amperes per square foot and the electrolysis is then continued for an additional periodabout22 minutes. The deposit formed initiallyon the cathode is alfilm of red selenium which is transformed very rapidly to the metallic formof selenium as the electrolysis After this electrolysis is completed, the-cathode is removed from the velectrolyte and is observed to be coated with an adherent deep black velvet-like deposit comprising microscopic Example 2.

a ,The process set forth in Example 1 is repeated with-themodification that the current density ,duringthe electrolysis ,ismaintained at 10 amperes per square foot of cathode surface for a sufficient time to produce .adeposit of equivalent thickness, i. e. forabout 2,30,1ninutes. After this electrolysisis completed, the cathode is removed from the electrolyte and is observed to be coated with anadherent deposit which does not have Examples The process set .forthin Example 1 is repeated except that-the electrolyte .maintained at a more elevated temperature, e. g. 125 C. or higher. During about the first 10 minutes of electrolysis the initially deposited red film willhave developed a comparatively smallnumber of isolated nuclei of the grey form and thereafter the deposit-will form preferentially on these spots. As the electrolysis proceeds, additional nuclei will form at a slow rate while the deposit becomes increasingly thicker on the spots already transformed to the grey metallic form. The electrolysis must therefore be continued for a substantial period of time before the red film is transformed and the resulting structure will be one of peaks and valleys, these phenomena being increasingly manifested with increase in operating temperature. From the foregoing it will be evident that operation of the presently-invented process under these conditions requires toolong a period of time to be wholly satisfactory for commercialuse.

Example :4

The process set forth in Example 1 is repeated except that the electrolyte is maintained at a less elevated temperature, e. g. C., or lower. During the first 10 minutes ofelectrolysis the initially deposited red film will bepunctured at spots due to the evolution of hydrogen gas bubbles and while the red film will gradually transform to grey metallic selenium, theresultant deposit will be pitted, the bits'usually extending through to the base plate, the phenomena being increasingly manifest with decrease in temperatureof the electrolysis. .From the foregoingit will be evident that operationofthe presently invented process under these conditions does not yield a satisfactory coatin'g for-use in rectifier.

The process setforth in Example 1 is repeated with the modification that the electrolyte .containsno sulfuric acid. After formation of the initial red film, the resistance of the bath will increase considerably requiring considerable .volt age increases to maintain the specified current density. Further operation will be accompanied by vigorous hydrogen ,eivolutionand a .continuous adherent deposit willnot be obtained.

Example 6- The processas set forth in Example 1 is repeated with themodification that a chemically equiv.- alent quantity of pure orthophosphorio acid is substituted for the sulfuric acid therein, ,employed." After formation of the initial red film, the resistance of the bath willincrease consider: ably requiring considerable voltage increases to maintain the specified current density. Further operation will be accompanied by vigorous hydrogen. evolution and a continuous adherent deposit will not .be obtained.

Example? The process asset forth in Example 1 is repeated with the modification thatthe electrolyte usedis composed of orthophosphoric acid saturated at a' temperatureof about -C. with selenium dioxide and following the initial electrolysis for 10 minutes at a current density of about 10 amperes per square foot of cathode surface, the electrolysis is then' continued for an additional period of about 29 minutes using a current density of about 75 amperes per square foot of cathode surface. The product obtained after these operations has substantially the same appearance as that of the product obtained by fac ticeof theprocess ofExample 1.

Example-8 The process of Example'l i s-repeated with the modification that a" cliem-ically equivalent quantity'of selenic acid is -"s'ubstit'uted"for' the sulfuric acid therein employed. The product obtained by these operations has the same acicular mat structure and properties as that obtained by practice of the process of Example 1.

Example 9 The process of Example 1 is repeated with the modifications that instead of a platinum cathode a cathode of pure annealed sheet nickel is used during the electrolysis and this cathode is al- 75 lowed to remain in the electrolyte for a short 11 period, say -30 seconds, prior to applications 'of the electrical potential; Apart from this difference in the metal base plate used, the product is identical With that obtained by practice of the process of Example 1..

Example 10 deposit of selenium is obtained and a rectifier madetherefrom will have a lower resistance in both the more readily conducting and in the less readily conducting directions.

. Example 1 1 The process of Example}! is repeated with the modification that the cathodeis an aluminum shape having a non-porous coating of nickel electroplated thereon; The product obtained by practice of this modified process is identical with that'obtained in the process of Example 10, except for the difference position. 7

. It will be apparent to those versed in the art to Which this invention'pertains that modifications can be made in the described processes without thereby departing: from the spirit and substance of this invention;

Having thus described theinvention, what it is desired to secure by Letters Patient is:

1. An article of manufacture including an electrically conductive surface andv a substan tially continuous ssentiaLllyfnoh-porous coating upon said surface that comprises a mat substantially entirely composed of acicular microcrystallites of metallic gray selenium.

2. An article of manufacture including an electrically conductive surface and a substantially continuous, essentially non-porous coating upon said surface, dark gray to black in color and having a pile structure resembling velvet, substantially entirely comprising acicular microcrystalline gray metallic selenium. 3, An articleof manufacture includingan electrically conductive surface and a substantially continuous, essentially non-porous coating upon said surface that comprises gray crystalline metallic selenium directly electrodeposited thereon. V V V 4. An article of manufacture including an electrically conductive surface and a substantial ly continuous, essentially. non-porous coating in the base plate com' upon said surface, dark gray to black in color,

and having a pile structure resembling velvet, comprising gray crystalline metallic selenium directly electrodeposited upon said surface.

5. An article of manufacture including an electrically conductive surface and a coating thereon, said coati g compr s g a t a i s y adherent, non-porous substantially black pilelike mat, free from discontinuities-and composed of acicular microcrystallites of metallic selenium.

6. An article of manufacture including an electrically conductive surface and a coating thereon, said coating being substantially continuous, substantially black, non-porous, tenaciously adherent to said surface and being composed of acicular gray microcrystallites of metalhc selenium directly electrodeposited upon said surface.

7. An article of manufacture including an electrically conductive surfacev and a coating thereon, said coating comprising a tenaciously adherent, non-porous substantially black pile like mat, free from discontinuities and composed of acicular micro-crystallites of metallic selenium, substantially all of the micro-crystallites being oriented with their C axes at least nearly normal to said surface.

8. An alternating electric current rectifier that comprises a substantially continuous, non-porous and tenaciously adherent layer of gray acicular micro-crystalline metallic electrodepositedv sele: nium.

9. An alternating electric current rectifier that comprises a tenaciously adherent, non-porous pile-like mat, free from discontinuities and composed of acicular micro-crystallites of metallic selenium.

10. The process that comprises electrolytically depositing gray' crystalline, metallic selenium upon an electrical conductor"'as the cathode during the electrolysis of a highly concentrated aqueous solution of selenium dioxide having'a high concentration oftsulphuric acid dissolved therein, using a current density of approximately 10'amperes per square foot'of cathode surface during the initial period of electrolysis and thereafter increasing the 'current density to cause discharge of gray'crystalline metallic selenium at the desired rate without evolution of hydrogen at the cathode, at least the initial periodofsaid electrolysis being conducted'at a temperature-in the range of about C. to about C.

' ARTHUR; voN I-IIPPEL.

'MORTIMER C. BLOOM;

U REFERENCES cu we The following"references'are of record in the file of this patent:

UNITEDSTATES 'PA'I'ENTS' Number .Date

Name 1,743,160 Presser Jan; 14, 1930 2,030,443 Geisler Feb. 11, 1936 2,378,513 Thompson et al. June 19, 1945 Bloomr 1Jan...21,.1947 

10. THE PROCESS THAT COMPRISES ELECTROLYTICALLY DEPOSITING GRAY CRYSTALLINE METALLIC SELENIUM UPON AN ELECTRICAL CONDUCTOR AS THE CATHODE DURING THE ELECTROLYSIS OF A HIGHLY CONCENTRATED AQUEOUS SOLUTION OF SELENIUM DIOXIDE HAVING A HIGH CONCENTRATION OF SULPHURIC ACID DISSOLVED THEREIN, USING A CURRENT DENSITY OF APPROXIMATELY 10 AMPERES PER SQUARE FOOT OF CATHODE SURFACE DURING THE INITIAL PERIOD OF ELECTROLYSIS AND THEREAFTER INCREASING THE CURRENT DENSITY OF CAUSE DISCHARGE OF GRAY CRYSTALLINE METALLIC SELENIUM AT THE DESIRED RATE WITHOUT EVOLUTION OF HYDROGEN AT THE CATHODE, AT LEAST THE INITIAL PERIOD OF SAID ELECTROLYSIS BEING CONDUCTED AT A TEMPERATURE IN THE RANGE OF ABOUT 75* C. TO ABOUT 125* C. 